Mast cells, basophils, and eosinophils are three additional types of myeloid cells that play roles in innate and adaptive immune responses. All three share the common property of having cytoplasmic granules filled with various inflammatory and antimicrobial mediators, which are released from the cells on activation. Another common feature of these cells is their involvement in immune responses that protect against helminths and reactions that cause allergic diseases.

Mast Cells

Mast cells are bone marrow–derived cells that are most abundant in the skin and mucosal epithelia; on activation, they release many potent inflammatory mediators that defend against infections by helminthic parasites or cause symptoms of allergic diseases. A cytokine called stem cell factor (or c-KIT ligand) is essential for mast cell development. Normally, mature mast cells are not found in the circulation but are present in tissues, usually adjacent to small blood vessels and nerves (Fig. 1B). Their cytoplasm contains numerous membrane-bound granules, which are filled with preformed inflammatory mediators, such as histamine, and acidic proteoglycans that serve to store some mediators and bind basic dyes, imparting a dark blue color to the granules when special stains are used. Various stimuli can activate mast cells to release their cytoplasmic granule contents into the extracellular space, as well as to synthesize and release cytokines and inflammatory lipid mediators. The released histamine and other mediators promote changes in the blood vessels that cause inflammation. Mast cells express high-affinity plasma membrane receptors for a type of antibody called IgE and are usually coated with these antibodies. When the IgE antibodies on the mast cell surface bind to antigen, signaling events are induced that lead to the activation of the cell. Mast cells are also activated when they recognize microbial products, independent of IgE, and in this way they function as tissue sentinels of the innate immune system.

Fig1. Morphology of neutrophils, mast cells, basophils, and eosinophils. (A) The light micrograph of a Wright-Giemsa–stained blood neutrophil shows the multilobed nucleus, because of which these cells are also called polymorphonuclear leukocytes, and the faint cytoplasmic granules. (B) The light micrograph of a Wright-Giemsa–stained section of skin shows a mast cell (arrow) adjacent to a small blood vessel, identifiable by the red blood cell in the lumen. The cytoplasmic granules in the mast cell, which are stained purple, are filled with histamine and other mediators that act on adjacent blood vessels to promote increased blood flow and delivery of plasma proteins and leukocytes into the tissue. (C) The light micrograph of a Wright-Giemsa–stained blood basophil shows the characteristic blue-staining cytoplasmic granules. (D) The light micrograph of a Wright-Giemsa–stained blood eosinophil shows the characteristic segmented nucleus and red staining of the cytoplasmic granules. B, Courtesy Dr. George Murphy, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts. C, Courtesy Dr. Jonathan Hecht, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts.

Basophils

Basophils are blood granulocytes with many structural and functional similarities to mast cells. Like other granulocytes, basophils are derived from hematopoietic precursors, mature in the bone marrow (from progenitors distinct from those of mast cells), and circulate in the blood. Basophils constitute less than 1% of blood leukocytes (see Table 1). Although they are normally not present in tissues, basophils may be recruited to some inflammatory sites. These cells also contain granules that bind basic dyes (Fig. 1C), and they are capable of synthesizing many of the same mediators as mast cells. Like mast cells, basophils express IgE receptors, bind IgE, and can be triggered by antigen binding to IgE. Because basophil numbers are low in tissues, their importance in host defense and allergic reactions is uncertain.

Table1. Normal Blood Cell Counts

Eosinophils

Eosinophils are granulocytes that express cytoplasmic granules containing enzymes that are harmful to the cell walls of helminthic parasites but can also damage host tissues. Eosinophil granules contain mainly basic proteins that bind acidic dyes, such as eosin, and thus appear red in stained blood smears and tissue sections (Fig. 1D). Eosinophils are bone mar row–derived and circulate in the blood, from where they may be recruited into tissues. The cytokines GM-CSF, interleukin-3 (IL-3), and IL-5 promote eosinophil maturation from myeloid precursors. Various membrane receptors on eosinophils, including Fc receptors for IgA and IgG, TLRs, and IL-5 receptors, can generate signals that activate the cells to release their granule contents. Some eosinophils are normally present in peripheral tissues, especially in mucosal linings of the respiratory, gastrointestinal, and genitourinary tracts, and their numbers can increase by recruitment from the blood in the setting of inflammation.

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Major Features of Cell-Mediated Immunity

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